A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a stable and well-regulated output, switched-mode power converters are frequently employed to advantage. Switched-mode power converters generally include an inverter, a transformer having a primary winding coupled to the inverter, an output rectifier coupled to a secordary winding of the transformer, an output filter and a controller. The inverter generally includes a power switch, such as a field-effect transistor (FET), that converts an input voltage to a switched voltage that is applied across the transformer. The transformer may transform the voltage to another value and the output circuit generates a desired voltage at the output of the converter. The output filter typically includes an inductor and an output capacitor. The output capacitor smooths and filters the output voltage for delivery to a load.
There are two common methods of regulating the output voltage of the converter, namely, voltage-mode control and current-mode control. In voltage-mode control, the controller typically includes an error amplifier coupled to the output of the power converter. The controller further includes a modulator coupled between the error amplifier and the power switch. The error amplifier monitors the output voltage of the power converter and generates an error signal representing a deviation between the actual output voltage and a desired output voltage. The modulator then generates a drive signal for the power switch based on the error signal. For example, the drive signal may maintain the power switch in a conduction mode while the error signal exceeds an internal timing signal. The drive signal may then transition the power switch to a non-conduction mode when the periodic timing signal reaches the error signal.
In current-mode control, a current in the power converter, such as a switch current through the power switch or an inductor current through the output inductor, is substituted for, or added to, the periodic ramp signal. The output voltage of the converter is still fed back through the error amplifier circuit to provide a component of the error signal for the modulator. The aforementioned methods and variations thereof are widely used and are adequate for many loads.
In conventional voltage or current mode control, the switching cycle may be divided into a primary interval D (during which the power switch is in the conduction mode) and an auxiliary interval 1-D (during which the power switch is in the non-conduction mode). The modulator determines when the power switch will be conducting or non-conducting in concert with the internal timing signal and the error signal derived from the voltage or current being monitored. The modulator thus exhibits an active decision process during the primary interval D. Any change in the error signal either continues to keep the power switch in the conduction mode or places the power switch in the non-conduction mode, thereby ending the primary interval D. During the auxiliary interval 1-D, however, the modulator typically waits for the timing circuit to begin a new switching cycle.
A converter employing an output circuit organized as a current doubler is of particular interest. The output current doubler circuit uses two inductors that are arranged to deliver current in a balanced fashion to an output load. The output current is the sum of the two inductor currents. Under ideal conditions, the ratio of the inductances of the two inductors would be selected to equal D/(1-D), which would provide zero ripple current for almost all values of load current. Practically, the inductances vary over a range, depending on acceptable design tolerances. Therefore, the two inductor ripple currents are typically different in value.
For larger values of load current, this difference in inductor ripple currents causes no appreciable problem. However, for a load current that is near zero, the imbalance in inductor currents induces a current into the output capacitor often causing the modulator to greatly reduce its duty cycle thereby increasing losses in the converter. This behavior is conventionally avoided by "preloading" the output. Preloading is accomplished by applying a minimum load to the converter's output thereby forcing the converter to deliver a minimum amount of load current. The preload current must be sufficient to accommodate the imbalance in the inductor ripple currents. Unfortunately, the preload current reduces the overall efficiency of the converter and generates additional heat.
Accordingly, what is needed in the art is an effective way to minimize the value of preload current for a converter employing an output current doubler circuit.